The invention is directed generally to improvements in color television receivers, and specifically to an improved tint control circuit for use in such receivers.
Conventionally, color television receivers include two or more color demodulators for demodulating the color components of a broadcast television signal. Such demodulators usually receive the color portion of the television signal and at least one color subcarrier input. The two demodulator inputs are essentially multiplied together to provide a demodulated output which varies at a function of the phase difference between the inputs. The demodulated output signal is then employed to modulate currents in a cathode ray tube to produce color images.
To vary the hue or tint of the color image, it is conventional to include a manually operable tint control knob on the receiver to permit a viewer to alter the phase of the subcarrier input to the demodulators. Typically, rotation of the knob varies a D.C. control voltage which is applied to a tint control circuit so as to vary the proportions of a pair of subcarrier currents of quadrature phases which are summed and then applied to a demodulator as a resultant subcarrier current.
Because the phase of the resultant subcarrier current varies as a function of the relative proportions of the summed currents, the hue of the demodulated color output varies accordingly.
Although such tint control circuits do perform satisfactorily in a general sense, many of them have certain drawbacks. For example, some such tint control circuits operate on or develop subcarrier signals which have rectangular type waveforms as opposed to sinusoidal waveforms. The demodulators which such tint control circuits drive are typically sensitive to the subcarrier's zero-crossing point, and the rectangular type subcarrier waveforms tend to have ambiguous zero-crossing points. Hence, the subcarrier outputs of the tint control circuit have been first filtered so as to drive the demodulators with the subcarrier's fundamental component to remove the ambiguity associated with the subcarrier's zero-crossing point. Not only does such filtering add extra expense, it also usually precludes the desired D.C. coupling between the tint control circuit and the demodulators.
A further drawback of some conventional tint control circuits is their inability to provide a pair of subcarrier outputs whose phases are separated by more than ninety degrees to render flesh tones more reproducible. In addition, the phase separation which has been provided may fluctuate undesirably as the manual tint control knob is adjusted over its entire range.
Prior tint control circuits generally include one or more of the problems mentioned above and are, therefore, less than perfectly satisfactory.